Metal containers for foods and beverages generally have one or more coatings to inhibit contact of the packed product and/or process water or steam onto the metal in order to prevent or minimize corrosion of the metal. Any such corrosion will affect the quality of the packed product or influence the aesthetic appearance of the container that, in long term, will affect the quality of the packed product. In the production of metal containers of this kind, there is often provided metal in the form of sheets or coils that is pre-coated with a suitable coating and which is subsequently processed to obtain the desired container by shaping (such as for three-piece can production) or deformation (such as for deep drawing processes). In producing cans for food and beverages, coatings are required that have no or low toxicity and that are flexible enough to withstand deformations during the can manufacturing process and abrasion/scratches during transportation. Furthermore, filled food cans are often sterilized under application of temperatures up to 135° C. The coating must therefore be sufficiently stable at those temperatures and must be capable of maintaining good adhesion to the surface of the metal.
Various types of coating compositions are well known in the can coating industry. Such coating compositions include epoxy resins, polyester resins, polyvinylchloride (PVC) resins, acrylic resins, and epoxy esters as main resin components. Such coating compositions also include a cross-linking agent such as a phenolic or an amino resin, such as those based upon urea, melamine, benzoguanamine or hexamethoxy methyl melamine, depending on the required application.
For external can coating applications, the most widely used coating systems are generally based upon either an epoxy/phenolic or an epoxy/amino system. Epoxy/phenolic formulations have some disadvantages with respect to color retention on multiple bakes and high pH process resistance. Epoxy/amino resin systems generally have somewhat better process resistance than the epoxy/phenolic systems, but have some disadvantages with respect to flexibility and formability.
The use of blocked polyisocyanates as cross-linkers or modifiers has become increasingly popular because of their performance attributes, particularly with respect to process resistance and flexibility. However, compliance to food regulations is an issue with this particular cross-linker. Typically, the blocked polyisocyanate systems use saturated polyester resin as the main binder. Some such systems may optionally include an amino resin, a phenolic resin or a small amount of epoxy resin. These coatings are generally recommended for use on can exteriors where there is no direct food contact due to the residual content of free blocking agent from the polyisocyanate.
Process resistance is a major requirement for coating compositions used to coat the exterior surfaces of food cans. Process water varies in pH, from neutral (pH 6-8) up to as high as pH 10-12. Process resistance (i.e., ability of the coating to withstand high temperature, high pressure sterilization conditions) is inversely proportional to film flexibility and formability. In other words, if process resistance is improved, it negatively affects the film flexibility/formability and vice versa. This has always been the performance trade-off observed on current external can coating systems. Process resistance also influences the resistance to bi-metallic effect of the can coating. Bi-metallic corrosion occurs when different metals are in contact in the presence of an electrolyte (e.g., EOE ends made of tinplate or electro chromium coated steel (ECCS) with aluminum tabs with retort water as an electrolyte). The coating for this type of application must possess good process resistance to block the flow of current from one metal to the other.
It is an object of the invention to provide a high pH process resistant can coating composition that also exhibits good film flexibility/formability and scuff resistance to withstand the mechanical stresses of can forming. In addition, it is also an object to provide a can coating composition with improved bi-metallic corrosion resistance after processing. It is a further object to provide a can coating composition that complies with FDA 175.300 regulations so that it may also be used as an internal can coating that comes in direct contact with dry food.